JP2016017200A - Die steel and warm/hot-working die - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a die steel that has a high crack resistance and wear resistance, and moreover, that is low in cost, and to provide a warm/hot-working die using the same.SOLUTION: A die steel contains 0.30<C<0.45 mass%, 0.01<Si<0.15 mass%, 0.50<Mn<1.18 mass%, 0.015<P<0.050 mass%, 5.36<Cr<5.92 mass%, 0.98<Mo<1.37 mass%, 0.30<V<0.72 mass%, and 0.0150<N<0.0800 mass%, and the remainder is composed of Fe and inevitable impurity. The die steel may contain, as the inevitable impurity, a predetermined amount or less of S, Cu, Ni, Al, W, O, Co, Nb, Ta, Ti, Zr, B, Ca, Se, Te, Bi, Pb, or Mg.SELECTED DRAWING: Figure 11

Description

  TECHNICAL FIELD The present invention relates to a mold steel and a hot mold, and more particularly, to a mold steel that is low-cost, hard to crack, and excellent in wear resistance, and a hot mold using the same. .

  A mold used for hot-to-warm plastic working (warm mold) is usually manufactured by quenching and tempering a material, and forming a recess of a desired shape in the material by die-sculpting. Yes. Further, when plastic working is performed hot to warm using such a mold, the mold receives a large heat cycle and a large load. Therefore, the material used for the hot mold is required to be excellent in toughness, high temperature strength, wear resistance, crack resistance, heat check resistance and the like. However, in general, it is difficult to improve a plurality of properties at the same time in mold steel.

In order to solve this problem, various proposals have heretofore been made.
For example, Patent Document 1 discloses a hot work tool steel in which, in addition to optimizing the components, the amount of inclusions and the directionality of the martensite structure are controlled.
This document describes that by controlling the direction of martensite, the variation in machinability is reduced and the progress of cracks is suppressed.

Patent Document 2 discloses a crushing blade that is not steel for molds, but in addition to optimizing the components, the amount of retained austenite is reduced to 1.0% or less by high-temperature tempering.
This document describes that, by setting the retained austenite to 1.0% or less, secular change accompanied by expansion due to decomposition of the retained austenite is suppressed.

Patent Document 3 discloses a hot forging die in which an oxide film is formed on the surface by heat treatment in addition to optimizing components.
This document describes that by forming an oxide film on the mold surface, plastic flow of the mold is suppressed, and wear resistance is improved.

Patent Document 4 discloses a hot working die in which a nitride layer having a predetermined depth and a predetermined hardness is formed on the surface in addition to optimizing the components.
In the same document,
(A) Abrasion resistance is improved by increasing the depth of the nitride layer, and
(B) It is described that heat cracks can be prevented by adjusting the hardness of the nitride layer to an appropriate value.

Patent Document 5 discloses a steel for hot forging die in which alloy elements such as Ni, Mo, Si, Cr, Mo, and V are optimized.
In the same document,
(A) The addition of Ni and Mo is effective in improving toughness, and
(B) It is described that the addition of Si, Cr, Mo, and V is effective in improving the high temperature strength.

Furthermore, Patent Document 6 discloses a hot work tool steel in which an alloy element such as B or Al is optimized.
In the same document,
(A) The addition of B improves high temperature strength without reducing toughness, and
(B) In order to make the B addition effect more effective, it is described that a small amount of Al is essential.

Especially for hot molds,
(A) Lower mold cost,
(B) avoiding cracks that occur during processing (so-called “large cracks” that originate from the corners of the recesses and penetrate the mold);
(C) suppression of wear during processing;
3 points are required.

Among these, the importance of wear control is increasing. The reason for this is that the mold is continuously used by repairing the wear by “building up welding”, “surface down” or the like while avoiding cracks.
Therefore, how to suppress wear after making it difficult to crack is an important issue in securing the mold life. Conventionally, in order to suppress wear, measures have been taken to increase the high-temperature strength by adding V, Mo, or the like. Naturally, this increases the material cost and makes it difficult to reduce the cost. Furthermore, since the larger the mold, the harder it becomes to enter (harder to martensite) and the toughness decreases, so there is a limit to avoiding cracks by the conventional method. That is, it becomes difficult to realize the essential requirement of “difficult to break”.

JP 2003-268486 A JP 2007-297691 A JP 2008-308745 A JP 2010-065280 A Japanese Patent Laid-Open No. 06-256897 JP 08-269625 A

The problem to be solved by the present invention is to provide a mold steel having high cracking resistance and wear resistance and a low cost, and a hot mold using the same.
Another problem to be solved by the present invention is to provide a mold steel exhibiting high crack resistance and wear resistance and a hot mold using the same even when applied to a large mold. There is to do.

In order to solve the above problems, the mold steel according to the present invention is:
0.30 <C <0.45 mass%,
0.01 <Si <0.15 mass%,
0.50 <Mn <1.18 mass%,
0.015 <P <0.050 mass%,
5.36 <Cr <5.92 mass%,
0.98 <Mo <1.37 mass%,
0.30 <V <0.72 mass% and 0.0150 <N <0.0800 mass%
The balance consists of Fe and inevitable impurities.

  The hot mold according to the present invention is made of the mold steel according to the present invention.

The mold steel according to the present invention can simultaneously achieve cost reduction, crack avoidance, and wear suppression. this is,
(A) Because the amount of expensive rare elements such as Mo and V is reduced,
(B) Since the hardenability was improved by optimizing the amount of Mn and Cr, and
(C) It is considered that the amount of Si was reduced and the high-temperature strength and thermal conductivity were improved.

It is a figure which shows the influence of Si which exerts on high temperature strength. It is a figure which shows the influence of Si which acts on thermal conductivity. It is a figure which shows the influence of Mn which gives to an impact value (toughness). It is a figure which shows the influence of Mn which acts on thermal conductivity.

It is a figure which shows the influence of P which has on high temperature strength. It is a figure which shows the influence of P exerting on an impact value (toughness). It is a figure which shows the influence of Cr which gives to an impact value (toughness). It is a figure which shows the influence of Cr which acts on thermal conductivity.

It is a figure which shows the influence of the heat conductivity which acts on metal mold | die surface temperature. It is a figure which shows the influence of N which acts on (gamma) crystal grain diameter. It is a figure (image figure) which shows the relationship between the temperature and intensity | strength of steel for metal mold | die which concerns on this invention, and conventional steel.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Steel for molds]
The mold steel according to the present invention contains the following elements, with the balance being Fe and inevitable impurities. The kind of additive element, its component range, and the reason for limitation are as follows.

[1.1. Main constituent elements]
(1) 0.30 <C <0.45 mass%:
When the amount of C is small, the hardness is insufficient. In order to obtain the necessary hardness (30 to 57 HRC) as a mold by quenching and tempering, the amount of C needs to be more than 0.30 mass%. The amount of C is more preferably more than 0.32 mass%, more preferably more than 0.33 mass%.
On the other hand, when the amount of C becomes excessive, the thermal conductivity decreases. Moreover, the carbide | carbonized_material which crystallizes at the time of the solidification after casting increases, and toughness falls. Therefore, the amount of C needs to be less than 0.45 mass%. The amount of C is more preferably less than 0.44 mass%, more preferably less than 0.43 mass%.

(2) 0.01 <Si <0.15 mass%:
When the amount of Si is small, the machinability is remarkably deteriorated. In order to obtain good machinability, the amount of Si needs to be more than 0.01 mass%. The amount of Si is more preferably more than 0.02 mass%, more preferably more than 0.03 mass%.
On the other hand, when the amount of Si becomes excessive, a decrease in high-temperature strength (see FIG. 1) and a decrease in thermal conductivity (see FIG. 2) become problems. Therefore, the amount of Si needs to be less than 0.15 mass%. The amount of Si is more preferably less than 0.14 mass%, and more preferably less than 0.13 mass%.

  In the present invention, expensive Mo and V are reduced for cost reduction. Mo and V are elements that contribute to the improvement of high-temperature strength. For this reason, in this invention, it is necessary to supplement or surpass the fall of the high temperature strength by the reduction | decrease of Mo and V by adjustment of another element. Therefore, attention was focused on Si in order to increase the high temperature strength.

(3) 0.50 <Mn <1.18 mass%:
When the amount of Mn is small, hardenability, that is, toughness is lowered (see FIG. 3). In order to obtain high toughness, the amount of Mn needs to be more than 0.50 mass%. The Mn is slightly higher than the Mn amount of 0.3 to 0.5 mass% of the steel for hot molds that is generally commercially available. The amount of Mn is more preferably more than 0.53 mass%, more preferably more than 0.56 mass%.
On the other hand, when the amount of Mn becomes excessive, the thermal conductivity decreases (see FIG. 4). Further, the toughness decreases depending on the tempering temperature and the type and amount of impurity elements. Therefore, the amount of Mn needs to be less than 1.18 mass%. The amount of Mn is more preferably less than 1.05 mass%, and more preferably less than 0.95 mass%.

(4) 0.015 <P <0.050 mass%:
P is effective for improving the high temperature strength. When the amount of P is small, the effect of increasing the strength is small (see FIG. 5). In order to obtain high high-temperature strength, the amount of P needs to be more than 0.015 mass%. The amount of P is more preferably more than 0.016 mass%, and more preferably more than 0.017 mass%.
On the other hand, when the amount of P becomes excessive, toughness decreases (see FIG. 6). Therefore, the amount of P needs to be less than 0.050 mass%. The amount of P is more preferably less than 0.040 mass%, and still more preferably less than 0.035 mass%.

(5) 5.36 <Cr <5.92 mass%:
If the amount of Cr is small, the high temperature strength, corrosion resistance, and hardenability (see FIG. 7) are insufficient. In order to improve the high temperature strength, corrosion resistance, and hardenability, the Cr amount needs to be more than 5.36 mass%. The amount of Cr is more preferably more than 5.38 mass%, and still more preferably more than 5.40 mass%.
On the other hand, when the amount of Cr becomes excessive, the thermal conductivity decreases (see FIG. 8). When the thermal conductivity is lowered, the surface temperature of the mold rises (see FIG. 9), and wear easily proceeds. Furthermore, when the amount of Cr is excessive, pearlite is likely to precipitate during quenching. When pearlite precipitates, the toughness decreases. Therefore, the Cr amount needs to be less than 5.92 mass%. The amount of Cr is more preferably less than 5.88 mass%, and still more preferably less than 5.84 mass%.

(6) 0.98 <Mo <1.37 mass%:
When the amount of Mo is small, it is difficult to secure hardness due to the secondary effect during tempering, and the high temperature strength becomes insufficient. In order to improve hardness and high-temperature strength, the Mo amount needs to be more than 0.98 mass%. The amount of Mo is more preferably more than 1.01 mass%, more preferably more than 1.04 mass%.
On the other hand, when the amount of Mo becomes excessive, the fracture toughness value decreases greatly. In addition, the material cost increases. Therefore, the Mo amount needs to be less than 1.37 mass%. The amount of Mo is more preferably less than 1.35 mass%, and more preferably less than 1.33 mass%.

(7) 0.30 <V <0.72 mass%:
When the amount of V is small, V-based fine particles (carbides and carbonitrides) remaining during quenching are reduced. For this reason, it becomes difficult to suppress the movement of the γ grain boundary, the γ crystal grains become coarse and the toughness decreases. Moreover, it is difficult to ensure hardness due to the secondary effect during tempering, and the high-temperature strength is insufficient. In order to improve toughness, hardness, and high-temperature strength, the V amount needs to be more than 0.30 mass%. The amount of V is more preferably more than 0.50 mass%.
On the other hand, when the amount of V becomes excessive, carbides that crystallize during solidification after casting increase and the toughness decreases. In addition, the material cost increases. Therefore, the V amount needs to be less than 0.72 mass%. The amount of V is more preferably less than 0.68 mass%.

(8) 0.0150 <N <0.0800 mass%:
When the amount of N is small, V-based fine particles (carbides and carbonitrides) remaining during quenching are reduced. Therefore, it becomes difficult to suppress the movement of the γ grain boundary, and the γ grain becomes coarse and the toughness is lowered (see FIG. 10). In order to improve toughness, the amount of N needs to be more than 0.0150 mass%. The amount of N is more preferably more than 0.0155 mass%, more preferably more than 0.0160 mass%.
On the other hand, when the amount of N is excessive, V-based coarse particles (carbides and carbonitrides) that crystallize during solidification after casting increase, which acts as a crack starting point and propagation path, and reduces toughness. Further, when the amount of N becomes excessive, the cost for adding nitrogen increases. Therefore, the N amount needs to be less than 0.0800 mass%. The N amount is more preferably less than 0.0700 mass%, more preferably less than 0.0500 mass%.

[1.2. Inevitable impurities]
Mold steel according to the present invention, as the inevitable impurities,
S ≦ 0.003 mass%,
Cu ≦ 0.30 mass%,
Ni ≦ 0.50 mass%,
Al ≦ 0.10 mass%,
W ≦ 0.10 mass%,
O ≦ 0.01 mass%,
Co ≦ 0.10 mass%,
Nb ≦ 0.004 mass%,
Ta ≦ 0.004 mass%,
Ti ≦ 0.004 mass%,
Zr ≦ 0.004 mass%,
B ≦ 0.0001 mass%,
Ca ≦ 0.0005 mass%,
Se ≦ 0.03 mass%,
Te ≦ 0.005 mass%,
Bi ≦ 0.01 mass%,
Pb ≦ 0.03 mass%, or
Mg ≦ 0.02mass%
May be included.

The mold steel according to the present invention may contain one or more elements as described above. When the content of the element is not more than the above upper limit value, the element behaves as an inevitable impurity.
On the other hand, a part of the element may be contained exceeding the upper limit. In this case, the following effects are obtained according to the type and content of the element.

[1.3. Sub-constituent elements]
In addition to the main constituent elements described above, the mold steel according to the present invention may further contain one or more elements as described below. The kind of additive element, its component range, and the reason for limitation are as follows.

(1) 0.50 <Ni ≦ 4.50 mass%:
In recent years, the size of molds tends to increase as parts become larger and integrated. Large molds are difficult to cool. For this reason, when a large mold made of a steel material with low hardenability is quenched, ferrite, pearlite, or coarse bainite precipitates during the quenching, and various properties deteriorate.

In general, the amount of Mn is often increased to improve the hardenability. However, if Mn alone is excessively increased, the toughness may decrease (tempering embrittlement) depending on the tempering temperature and the type and amount of the impurity element. For such a concern, Ni may be added to enhance the hardenability. For this purpose, the Ni amount is preferably more than 0.50 mass%. The amount of Ni is more preferably more than 0.53 mass%, more preferably more than 0.56 mass%.
On the other hand, when the amount of Ni becomes excessive, there are problems that the thermal conductivity is lowered, the cost is increased, and the softening by annealing is difficult and the machinability is lowered. Therefore, the amount of Ni is preferably 4.50 mass% or less. The amount of Ni is more preferably 3.30 mass% or less.

(2) 0.30 <Cu ≦ 2.00 mass%:
Since Cu has an effect of improving hardenability, Cu may be added. Cu also has the effect of increasing strength by aging precipitation. In order to obtain such an effect, the amount of Cu is preferably more than 0.30 mass%. The amount of Cu is more preferably 0.50 mass% or more.
On the other hand, when the amount of Cu is excessive, there is a problem that the thermal conductivity is lowered, the cost is increased, and the softening by annealing is difficult and the machinability is lowered. Therefore, the amount of Cu is preferably 2.00 mass% or less. The amount of Cu is more preferably 1.50 mass% or less.

(3) 0.10 <Al <1.50 mass%:
Al combines with N to form AlN, and has the effect of suppressing the movement of austenite grain boundaries (ie, grain growth). Al also has the effect of increasing the strength of steel by forming an intermetallic compound with Ni. Furthermore, Al contributes to precipitation hardening by forming a nitride, and thus has an effect of increasing the surface hardness of the nitrided steel material. It is effective to use a steel material containing Al for molds and parts that are subjected to nitriding for higher wear. In order to obtain such an effect, the amount of Al is preferably more than 0.10 mass%. The amount of Al is more preferably more than 0.11 mass%, more preferably more than 0.12 mass%.
On the other hand, when the amount of Al becomes excessive, there is a problem that the thermal conductivity is lowered and the toughness is lowered. Therefore, the amount of Al is preferably less than 1.50 mass%. The amount of Al is more preferably less than 1.40 mass%, and more preferably less than 1.30 mass%.

(4) 0.003 <S ≦ 0.250 mass%:
(5) 0.0005 <Ca ≦ 0.2000 mass%:
(6) 0.03 <Se ≦ 0.50 mass%:
(7) 0.005 <Te ≦ 0.100 mass%:
(8) 0.01 <Bi ≦ 0.50 mass%:
(9) 0.03 <Pb ≦ 0.50 mass%:
The mold steel according to the present invention has a slightly poor machinability due to a small amount of Si. In order to improve workability, it is preferable to add one or more of S, Ca, Se, Te, Bi, and Pb. Further, the amount of each element is preferably an amount exceeding the lower limit value.
On the other hand, when the amount of these elements is excessive, there is a problem that the mold material is easily cracked when hot working. Therefore, the amount of each element is preferably not more than the above upper limit value.

(10) 0.004 <Nb ≦ 0.100 mass%:
(11) 0.004 <Ta ≦ 0.100 mass%:
(12) 0.004 <Ti ≦ 0.100 mass%:
(13) 0.004 <Zr ≦ 0.100 mass%:
If the quenching heating temperature becomes high or the quenching heating time becomes long due to unexpected equipment troubles, etc., there is a concern about deterioration of various characteristics due to coarsening of crystal grains. In preparation for such a case, it is preferable to add one or more of Nb, Ta, Ti, and Zr. When these elements are added, coarsening of austenite crystal grains can be suppressed by fine precipitates formed by these elements. In order to obtain such an effect, the amount of each element is preferably an amount exceeding the above lower limit value.
On the other hand, when the amount of these elements becomes excessive, the precipitates become coarse, and there is a problem that the toughness is lowered as a starting point of fracture. Therefore, the amount of each element is preferably not more than the above upper limit value.

(14) 0.0001 <B ≦ 0.0050 mass%:
B has an effect of improving hardenability. In order to obtain such an effect, the amount of B is preferably more than 0.0001 mass%. The amount of B is more preferably more than 0.0002 mass%, more preferably more than 0.0003 mass%.
On the other hand, when the amount of B becomes excessive, the hardenability is lowered. Therefore, the amount of B is preferably 0.0050 mass% or less.
B can be added to improve hardenability or improve machinability. B is easy to combine with N in steel to form BN. In order to improve hardenability, since B exists alone in the steel, an appropriate amount of Nb, Ta, Ti, Zr, etc., which is easier to bond with N than B, is added, and N is fixed as these nitrides. To do. Note that Nb, Ta, Ti, Zr, etc. are not added, and even if B is combined with N in the steel to form BN, if excess B is present alone in the steel, it has hardenability. Increase.
In order to improve machinability, BN may be formed. BN is similar in nature to graphite and reduces cutting resistance while improving chip crushability.
In addition, when B and BN exist in steel, hardenability and machinability are improved simultaneously.

(15) 0.10 <W ≦ 5.00 mass%:
(16) 0.10 <Co ≦ 4.00 mass%:
An increase in the amount of C is effective for increasing the strength, but an excessive increase in the amount of C has the above-mentioned adverse effects. In order to increase the strength without incurring such problems, W and / or Co may be selectively added.

W increases the strength by fine precipitation of carbides. In order to obtain such an effect, the amount of W is preferably more than 0.10 mass%. The amount of W is more preferably 0.30 mass% or more.
On the other hand, when the amount of W becomes excessive, the cost increases. Therefore, the amount of W is preferably 5.00 mass% or less. The amount of W is more preferably 4.00 mass% or less.

Co increases the strength by solid solution in the base material, and at the same time contributes to precipitation hardening through changes in the form of carbide. In order to obtain such an effect, the amount of Co is preferably more than 0.10 mass%. More preferably, the amount of Co is 0.30 mass% or more.
On the other hand, when the amount of Co becomes excessive, there are problems that the cost increases and the thermal conductivity decreases. Therefore, the amount of Co is preferably 4.00 mass% or less. More preferably, the amount of Co is 3.00 mass% or less.

[2. Hot and hot mold]
The hot mold according to the present invention is made of the mold steel according to the present invention. The shape and size of the mold are not particularly limited, and can be arbitrarily selected according to the purpose. The mold steel according to the present invention is particularly suitable for a large mold because it has improved hardenability without reducing strength and toughness.

Specifically, the mold steel according to the present invention includes hot forging, sub-hot forging, warm forging, die casting, hot pressing of steel plates (also called hot stamping, die quenching, press quenching), plastic injection molding, and the like. It is suitable as a material used for a mold or a part thereof.
Moreover, the steel for metal mold | die which concerns on this invention can also be used as a welding material in the state of a bar and a wire. At that time, the chemical component of the mold to be repaired may be different from or within the range of the mold steel according to the present invention.
Further, it is also effective to combine a surface modification treatment (shot blasting, sand blasting, nitriding, PVD, CVD, plating, etc.) on the mold using the mold steel according to the present invention or its parts.

[3. Manufacturing method of steel for mold and hot mold]
The mold steel according to the present invention is:
(1) Melting and casting the raw materials blended so as to become predetermined components,
(2) After forging the ingot, hot forging,
(3) Perform normalization,
(4) It can be manufactured by annealing. In this case, it is preferable to select the optimum conditions for each step according to the composition of the mold steel.

When materials such as normal processing (melting → refining → casting → plastic processing → softening heat treatment) are subjected to machining, quenching and tempering, and finishing machining (so-called rough processing → tempering → Finishing), the hot mold according to the present invention or its parts can be obtained. In addition, after quenching and tempering a raw material that has undergone a normal manufacturing method, machining of the mold shape is directly performed (so-called cutting out from a pre-hardened material) to obtain the hot mold or its parts according to the present invention. It is done.
Or you may manufacture a metal mold | die or its components from the powder and board | plate material which consist of steel for metal mold | die which concerns on this invention by an additive manufacturing method.

[4. Action]
As mentioned above, the hot mold is
(A) Lower mold cost,
(B) Avoiding cracks (large cracks) during processing,
(C) suppression of wear during processing;
3 points are required.

  In the present invention, the amount of expensive rare elements such as Mo and V is reduced in order to reduce costs. The total amount of Mo and V of the present invention is 1.3 to 2.1 mass%. The total amount of Mo and V is as high as 1.8 to 2.7 mass% for JIS SKD61 and 2.4 to 4.3 mass% for commercially available high-performance steel.

  In the present invention, to prevent cracking, the hardenability is increased and the toughness is increased. In recent years, molds have become larger and the quenching speed has decreased. When the hardenability is lowered, coarse bainite is generated, so that the toughness is lowered and the mold is easily cracked. In the present invention, the hardenability is enhanced by optimizing the amounts of Mn and Cr. Therefore, high toughness can be obtained even when the quenching speed is lowered, that is, even in a large mold.

In the present invention, for "wear suppression",
(A) increase high temperature strength,
(B) suppress the temperature rise of the mold surface,
The following two measures were taken.
(A) is the same idea as before, but adopts a new technique of low Si. Conventionally, improvement in high-temperature strength has been realized by increasing the amount of Mo and V. In the present invention, the knowledge that the high-temperature strength increases when the amount of Si is reduced is utilized, and the high-temperature strength is successfully increased even if the amount of Mo or V is reduced.

(B) is a new idea that has not existed before. On the surface of the mold that comes into contact with the high-temperature work material, a part that becomes extremely hot is generated. Since the strength of the portion that has become high becomes low, wear becomes obvious.
On the other hand, if the temperature reached on the mold surface is lowered, the strength increases accordingly, so that wear can be suppressed. Using the knowledge that the ultimate temperature of the mold surface decreases as the thermal conductivity of the mold increases (see FIG. 9), a chemical component having a high thermal conductivity was selected. There has never been a concept of suppressing wear with high thermal conductivity.

  As described above, the three points required for a hot mold, that is, low cost, avoidance of cracking, and suppression of wear, were simultaneously realized. The mold steel according to the present invention has high hardenability and thermal conductivity, and is inexpensive.

In FIG. 11, the relationship (image figure) of the temperature and intensity | strength of the steel for metal mold | die which concerns on this invention, and conventional steel is shown. If the mold surface of a conventional steel having low strength and low thermal conductivity is in the state of point A, it is in a state where it is very low in strength and easily worn. On the other hand, in the mold using the mold steel according to the present invention having high strength and high thermal conductivity, the mold surface is in a point B state. That is, when compared at the same temperature, an increase in strength due to a decrease in the mold surface temperature is taken into account in addition to the high strength compared to the conventional steel. As a result, the strength is much higher than that of point A, and it is difficult to wear. With such a mechanism, wear of the mold steel according to the present invention is suppressed.
In addition to being a steel material with high high-temperature strength originally, the idea of lowering the ultimate temperature of the mold surface with high thermal conductivity and further increasing the strength to suppress wear is the novelty of the present invention.

(Example 1-1: Effect of Si on high temperature strength)
[1. Preparation of sample]
A steel test piece having 0.39C-0.81Mn-0.016P-5.73Cr-1.19Mo-0.58V-0.0192N as a basic component and varying the amount of Si was prepared. The test piece was quenched from 1030 ° C. and tempered to 45 HRC.

[2. Test method and results]
Using the obtained test piece, a tensile test was performed by heating and holding at 600 ° C. FIG. 1 shows the influence of Si on the high temperature strength. From FIG. 1, it can be seen that the strength is increased by reducing the Si amount. In order to increase the high-temperature strength, less Si is better.

(Example 1-2: Effect of Si on thermal conductivity)
[1. Preparation of sample]
Test pieces of steel having 0.44C-0.89Mn-0.017P-5.73Cr-1.21Mo-0.59V-0.0203N as a basic component and varying the amount of Si were prepared. This test piece was quenched from 1030 ° C. and tempered to 43 HRC.

[2. Test method and results]
Using the obtained test piece, the thermal conductivity was measured at 25 ° C. by a laser flash method. FIG. 2 shows the influence of Si on the thermal conductivity. As can be seen from FIG. 2, the thermal conductivity increases as the Si amount decreases. When the thermal conductivity is 27.0 W / m / K or more, the effect of suppressing the temperature rise on the mold surface in contact with the high temperature workpiece is large. FIG. 2 shows that in order to obtain a thermal conductivity of 27.0 W / m / K or more stably, it is sufficient to set Si <0.15 mass%.

(Example 1-3: Effect of Mn on toughness)
[1. Preparation of sample]
Steel test pieces having 0.37C-0.08Si-0.017P-5.52Cr-1.30Mo-0.60V-0.0187N as basic components and varying the amount of Mn were prepared. The test piece was quenched from 1030 ° C. and tempered to 48 HRC. For quenching, the cooling rate to 550 ° C. was set to 20 ° C./min, and the cooling rate from 550 ° C. to 100 ° C. was set to 0.5 ° C./min.

[2. Test method and results]
The impact value was evaluated at room temperature using the obtained test piece. FIG. 3 shows the influence of Mn on the impact value (toughness). FIG. 3 shows that the impact value increases as the amount of Mn increases. When the impact value is 20 J / cm ² or more, the risk of large cracks in the mold is considerably reduced. From FIG. 3, it is understood that 0.5 mass% <Mn should be satisfied in order to stably obtain an impact value of 20 J / cm ² or more.

(Example 1-4: Effect of Mn on thermal conductivity)
[1. Preparation of sample]
Test pieces of steel having 0.40C-0.14Si-0.018P-5.75Cr-1.23Mo-0.61V-0.0172N as basic components and varying the amount of Mn were prepared. This test piece was quenched from 1030 ° C. and tempered to 43 HRC.

[2. Test method and results]
Using the obtained test piece, the thermal conductivity was measured at 25 ° C. by a laser flash method. FIG. 4 shows the influence of Mn on the thermal conductivity. As can be seen from FIG. 4, the thermal conductivity increases as the amount of Mn decreases. When the thermal conductivity is 27.0 W / m / K or more, the effect of suppressing the temperature rise on the mold surface in contact with the high temperature workpiece is large. From FIG. 4, it is understood that Mn <1.18 mass% may be used in order to stably obtain a thermal conductivity of 27.0 W / m / K or more.

(Example 1-5: Effect of P on high temperature strength)
[1. Preparation of sample]
Test pieces of steel having 0.38C-0.09Si-0.81Mn-5.71Cr-1.02Mo-0.55V-0.0213N as a basic component and varying the amount of P were prepared. The test piece was quenched from 1030 ° C. and tempered to 45 HRC.

[2. Test method and results]
Using the obtained test piece, a tensile test was performed by heating and holding at 600 ° C. FIG. 5 shows the effect of P on the high temperature strength. From FIG. 5, it can be seen that the increase in high-temperature strength becomes apparent when 0.015 mass% <P. When Mo and V are reduced for cost reduction, the high temperature strength decreases. On the other hand, when P is added in addition to the above-described reduction of Si, the high temperature strength reduction accompanying the reduction of Mo and V can be compensated.

(Example 1-6: Effect of P on toughness)
[1. Preparation of sample]
Steel test pieces having 0.36C-0.09Si-0.76Mn-5.53Cr-1.25Mo-0.60V-0.0193N as basic components and varying the P content were prepared. The test piece was quenched from 1030 ° C. and tempered to 46 HRC. For quenching, the cooling rate to 550 ° C. was set to 20 ° C./min, and the cooling rate from 550 ° C. to 100 ° C. was set to 0.5 ° C./min.

[2. Test method and results]
The impact value was evaluated at room temperature using the obtained test piece. FIG. 6 shows the effect of P on the impact value (toughness). FIG. 6 indicates that the impact value decreases as the P amount increases. When the impact value is 20 J / cm ² or more, the risk of large cracks in the mold is considerably reduced. From FIG. 6, it can be seen that P <0.050 mass% is sufficient to stably obtain an impact value of 20 J / cm ² or more.

(Example 1-7: Effect of Cr on toughness)
[1. Preparation of sample]
Steel test pieces having 0.36C-0.09Si-0.83Mn-0.016P-1.12Mo-0.57V-0.0208N as a basic component and varying the Cr content were prepared. The test piece was quenched from 1030 ° C. and tempered to 48 HRC. For quenching, the cooling rate to 550 ° C. was set to 20 ° C./min, and the cooling rate from 550 ° C. to 100 ° C. was set to 0.5 ° C./min.

[2. Test method and results]
The impact value was evaluated at room temperature using the obtained test piece. FIG. 7 shows the effect of Cr on the impact value (toughness). FIG. 7 shows that the impact value increases as the Cr content increases. When the impact value is 20 J / cm ² or more, the risk of large cracks in the mold is considerably reduced. From FIG. 7, it can be seen that 5.36 mass% <Cr is sufficient to stably obtain an impact value of 20 J / cm ² or more. Cr is slightly higher than the Cr amount of 4.9 to 5.2 mass% of 5Cr-based hot and hot mold steels that are generally available on the market.

  Since the softening resistance is lowered, the increase in Cr may be disliked, but the high-temperature strength is rather increased with the increase in Cr. Softening resistance and high temperature strength are incompatible, and high temperature strength should be noted when discussing mold wear. Therefore, Cr increase is desirable not only in toughness but also in high temperature strength. The decrease in high temperature strength due to the reduction of Mo and V is compensated by the increase in strength due to the reduction of Si, addition of P, and the increase of Cr.

(Example 1-8: Effect of Cr on thermal conductivity)
[1. Preparation of sample]
Test pieces of steel having 0.40C-0.14Si-1.05Mn-0.017P-1.18Mo-0.57V-0.0201N as a basic component and varying the Cr content were prepared. This test piece was quenched from 1030 ° C. and tempered to 43 HRC.

[2. Test method and results]
Using the obtained test piece, the thermal conductivity was measured at 25 ° C. by a laser flash method. FIG. 8 shows the effect of Cr on the thermal conductivity. FIG. 8 shows that the thermal conductivity is increased by decreasing the Cr content. When the thermal conductivity is 27.0 W / m / K or more, the effect of suppressing the temperature rise on the mold surface in contact with the high temperature workpiece is large. From FIG. 8, it can be seen that Cr <5.92 mass% is sufficient to obtain a thermal conductivity of 27.0 W / m / K or more stably.

  FIG. 9 shows the influence of thermal conductivity on the mold surface temperature. FIG. 9 shows the result of measuring the maximum temperature in the vicinity of the mold surface with a thermocouple when a material heated to 1200 ° C. is forged. The thermocouple was installed at a depth of 1 mm from the mold surface. From FIG. 9, it can be seen that when the thermal conductivity is high, the mold surface temperature hardly rises. This is because the heat flowing from the workpiece is easily dissipated into the mold, and the heat does not easily stay on the surface.

(Example 1-9: Effect of N on γ crystal grain size)
[1. Preparation of sample]
Test pieces of steel having 0.35C-0.10Si-0.81Mn-0.017P-5.72Cr-1.18Mo-0.53V as a basic component and varying the amount of N were prepared. This test piece was kept at 1030 ° C. for 5 hours and then quenched to obtain a martensite structure.

[2. Test method and results]
The old γ grain boundary was revealed with the corrosive liquid, and the γ particle size was evaluated according to JIS G0551. FIG. 10 shows the influence of N on the γ crystal grain size. FIG. 10 shows that the crystal grain size increases as N increases (the crystal grains become finer). In the 0.35C-0.53V steel used for this evaluation, it is preferable that a crystal grain size of 6 or more is secured. From FIG. 10, it can be seen that 0.015 mass% <N may be obtained in order to stably obtain a particle size of 6 or more.

(Examples 2-1 to 2-10, Comparative Examples 2-1 to 2-5)
[1. Preparation of sample]
Table 1 shows the chemical composition of the steel used in the test. Although not described in the table, other elements may be included in the amounts defined as impurities in the present invention. Comparative Example 2-1 is hot die steel SKD61, Comparative Example 2-2 is hot die steel SKD7, and Comparative Examples 2-3 to 2-5 are hot die steels developed and sold independently by steel manufacturers. is there. In Comparative Examples 2-1 to 2-5, at least three elements are out of the scope of the present invention.

These steels were cast into a 7 ton ingot and homogenized at 1240 ° C. for 24 hours. Then, it was forged into a rectangular cross section by forging and cooled to 100 ° C. or lower. Thereafter, tempering was performed at 700 ° C. for 6 hours, followed by heating to 900 ° C. and cooling (annealing) to 600 ° C. at a cooling rate of 15 ° C./hr. Thereafter, it was allowed to cool to room temperature.
From this rectangular material, a block of 400 mm × 500 mm × 800 mm (about 1240 kg) was cut out for a forging die. Furthermore, the raw material for producing the test piece which evaluates toughness, high temperature strength, and heat conductivity was cut out from some rectangular materials.

[2. Test method]
[2.1. Basic characteristics survey]
As basic characteristics, toughness, high-temperature strength, and thermal conductivity were investigated.

[2.1.1. Toughness (impact value)]
The specimen for the toughness investigation was 43HRC by quenching and tempering from 1030 ° C. For quenching, the cooling rate to 550 ° C. was set to 20 ° C./min, and the cooling rate from 550 ° C. to 100 ° C. was set to 0.5 ° C./min.
The impact value was evaluated at room temperature. When the impact value is 20 J / cm ² or more, the risk of large cracks in the mold is considerably reduced. Therefore, as the determination, an impact value of 20 J / cm ² or more was “◯”, and an impact value of less than 20 J / cm ² was “x”.

[2.1.2. High temperature strength]
The test piece for high-temperature strength investigation was also tempered in the same manner as the impact test piece. However, the tempering hardness was 45 HRC.
Evaluation of high temperature strength was performed at 600 degreeC. The higher the high temperature strength is, the more the wear is suppressed. Therefore, as a judgment, a tensile strength of 850 MPa or more was evaluated as “◯”, 820 MPa or more and less than 850 MPa as “Δ”, and less than 820 MPa as “x”.

[2.1.3. Thermal conductivity]
The test piece for thermal conductivity investigation was also tempered to 43HRC by quenching and tempering in the same manner as the impact test piece.
The thermal conductivity was measured at 25 ° C. by a laser flash method. When the thermal conductivity is 27.0 W / m / K or more, the effect of suppressing the temperature rise on the mold surface in contact with the high temperature workpiece is large. Therefore, as a judgment, the thermal conductivity of 27 W / m / K or more is “◯”, 24 W / m / K or more and less than 27 W / m / K is “△”, and the less than 24 W / m / K is “X” .

[2.2. Hot forging test]
[2.2.1. Large crack avoidance and wear control]
The aforementioned block material (about 1240 kg) was heated to 1030 ° C. and quenched with oil cooling. It has been found from numerical simulation that the average cooling rate in the temperature range governing toughness is 0.5 to 1 ° C./min. After the tempered block material was tempered to 44HRC, a forged product-shaped recess was formed on the design surface by machining to obtain a forged die.
The material heated to 1200 ° C. was forged with this mold. The number of forging shots was set to a maximum of 1500, and if a large crack occurred from the corner of the recess before reaching that shot, the test was stopped and an “x” determination was made. When 1500 shots were reached without large cracks, the wear depth d of the mold was measured after determining whether the crack was large. d <0.5 mm was “◯”, 0.5 mm ≦ d <1.0 mm was “Δ”, and 1.0 mm ≦ d was “x”.

[2.2.2. Inexpensive]
The cost reduction was evaluated by the total amount of Mo and V, which are expensive rare elements. When the total amount of Mo and V is 2.1% or less, “◯” is given, and when over 2.1% is 2.8% or less, “Δ” is given, and over 2.8% is “x”.

[3. result]
[3.1. Basic characteristics survey]
Table 2 shows the results of toughness, high temperature strength, and thermal conductivity. Each of Comparative Examples 2-1 to 2-5 has “Δ” or “x” in any one or more of the evaluation items, and there is a problem in basic characteristics. On the other hand, in all of Examples 2-1 to 2-10, all three items are “◯”, and there is no problem in basic characteristics.
Assuming an actual hot forging die, toughness is a large crack, and high temperature strength and thermal conductivity are properties that affect wear. The reason why the high temperature strength and thermal conductivity affect the wear is as described with reference to FIG.

[3.2. Hot forging test]
Table 3 shows the results of the hot forging test. When attention was paid to the comparative example, large cracks occurred in addition to Comparative Example 2-3. Since Comparative Example 2-3 has good hardenability, it is considered that even if the quenching speed is low, the toughness is high and large cracks can be avoided. Large cracks correspond to the toughness values in Table 2.
Regarding wear suppression, Comparative Examples 2-2, 2-4, and 2-5 were good. The behavior of wear corresponds to the high temperature strength and thermal conductivity in Table 2, and the higher the high temperature strength and thermal conductivity, the more the wear is suppressed.

Thus, among the comparative examples, although SKD61 (Comparative Example 2-1) is a relatively inexpensive material, there are difficulties in large cracks and wear. Other comparative examples are expensive, but are excellent in either avoiding large cracks or suppressing wear.
In contrast, in all of Examples 2-1 to 2-10, all items were “◯”, and the excellent basic characteristics confirmed in Table 2 were reproduced as excellent practical performance even in hot forging. . By applying the present invention, the three reasons of low cost, large crack avoidance, and wear suppression are realized at the same time by reducing the amount of Mo and V, high toughness, high temperature strength, and thermal conductivity. Is high.

The high strength at high temperature was achieved by (a) securing the necessary minimum amounts of Mo and V, (b) reducing Si, (c) adding P, and (d) increasing Cr.
For high toughness,
(A) Increased hardenability by increasing the amount of Mn and Cr,
(B) Refinement of γ crystal grains during quenching by optimizing C, V, and N,
(C) Suppressing crystallization of V-based coarse carbides by optimizing C, V, and N;
(D) Do not add P excessively,
Realized by.
The high thermal conductivity was realized by (a) Si reduction and (b) not excessively increasing C, Mn and Cr.

  The embodiment of the present invention has been described in detail above, but the present invention is not limited to the above embodiment, and various modifications can be made without departing from the scope of the present invention.

The mold steel according to the present invention includes hot forging, sub-hot forging, warm forging, hot pressing of steel plates, plastic molds used for injection molding, parts thereof, or welding materials for repairing molds. It can be used as a material.
Molds and parts made of steel for molds according to the present invention can be combined with surface modification (shot blasting, sand blasting, nitriding, PVD, CVD, plating, etc.).

Claims (9)

0.30 <C <0.45 mass%,
0.01 <Si <0.15 mass%,
0.50 <Mn <1.18 mass%,
0.015 <P <0.050 mass%,
5.36 <Cr <5.92 mass%,
0.98 <Mo <1.37 mass%,
0.30 <V <0.72 mass% and 0.0150 <N <0.0800 mass%
Steel for molds, the balance being Fe and inevitable impurities. 0.50 <Ni ≦ 4.50mass%
The mold steel according to claim 1, further comprising: 0.30 <Cu ≦ 2.00 mass%
The mold steel according to claim 1 or 2, further comprising: 0.10 <Al <1.50 mass%
The steel for mold according to any one of claims 1 to 3, further comprising: 0.003 <S ≦ 0.250 mass%,
0.0005 <Ca ≦ 0.2000 mass%,
0.03 <Se ≦ 0.50 mass%,
0.005 <Te ≦ 0.100 mass%,
0.01 <Bi ≦ 0.50 mass%, and
0.03 <Pb ≦ 0.50 mass%
The mold steel according to any one of claims 1 to 4, further comprising one or more elements selected from the group consisting of: 0.004 <Nb ≦ 0.100 mass%,
0.004 <Ta ≦ 0.100 mass%,
0.004 <Ti ≦ 0.100 mass%, and
0.004 <Zr ≦ 0.100 mass%
The mold steel according to any one of claims 1 to 5, further comprising any one or more elements selected from the group consisting of: 0.0001 <B ≦ 0.0050 mass%
The steel for mold according to any one of claims 1 to 6, further comprising: 0.10 <W ≦ 5.00 mass%, and
0.10 <Co ≦ 4.00 mass%,
The mold steel according to any one of claims 1 to 7, further comprising any one or more elements selected from the group consisting of:   A hot mold using the mold steel according to any one of claims 1 to 8.

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